Image processing apparatus and image forming apparatus

ABSTRACT

An image processing apparatus generates and outputs a binarized output pixel value corresponding to an input pixel value; and includes a filter processing unit, a quantizer, and a buffer. The quantizer is configured to quantize a pixel value after a filter process performed by the filter processing unit and thereby generate the output pixel value. The buffer is configured to store for each pixel a difference between (a) a sum of the pixel value after the filter process and the input pixel value and (b) the output pixel value. Further, the filter processing unit performs a bandpass filter process on the basis of the difference stored by the buffer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to and claims priority rights from JapanesePatent Application No. 2019-133182, filed on Jul. 18, 2019, the entiredisclosures of which are hereby incorporated by reference herein.

BACKGROUND 1. Field of the Present Disclosure

The present disclosure relates to an image processing apparatus and animage forming apparatus.

2. Description of the Related Art

When binarizing an input pixel value, an image processing apparatusperforms quantization (i.e. binarization) with error diffusion of avalue obtained by subtracting (a) a value obtained by applying a spatialbandpass filter to an output pixel value from (b) a value obtained byapplying a spatial lowpass filter to the input pixel value.Consequently, output dots after the binarization are clustered with acluster size (i.e. a condensing degree of output dots) corresponding toa filter characteristic of the bandpass filter.

Clustering the output dots gains granularity in some images to beoutputted. Therefore, in order to restrain the granularity in a coloroutput image, an image processing apparatus (a) performs a bandpassfilter process for differences between output pixel values and inputpixel values of adjacent pixels to a target pixel, (b) performs an errordiffusion process of a quantization error for an output of this filterprocess, and (c) performs quantization for a value obtained by the errordiffusion process; and thereby determines an output pixel value of thetarget pixel. In this filter process, the bandpass filter process isperformed with different characteristics in a primary scanning directionand in a secondary scanning direction and with different characteristicsin component colors of the image, and in order to restrain banding, afilter coefficient in the bandpass filter process is corrected with acorrection coefficient corresponding to an angle to the primary scanningdirection for each of the component colors.

However, the aforementioned image processing apparatus requires (a) abuffer for storing quantization errors in the error diffusion processand (b) a buffer for storing the differences between the output pixelvalues and the input pixel values in the filter process, and therefore arelatively large memory area is required for these two buffers.

SUMMARY

An image processing apparatus according to an aspect of the presentdisclosure is an image processing apparatus that generates and outputs abinarized output pixel value corresponding to an input pixel value; andincludes a filter processing unit, a quantizer, and a buffer. Thequantizer is configured to quantize a pixel value after a filter processperformed by the filter processing unit and thereby generate the outputpixel value. The buffer is configured to store for each pixel adifference between (a) a sum of the pixel value after the filter processand the input pixel value and (b) the output pixel value. Further, thefilter processing unit performs a bandpass filter process on the basisof the difference stored by the buffer.

These and other objects, features and advantages of the presentdisclosure will become more apparent upon reading of the followingdetailed description along with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram that indicates a configuration of an imageprocessing apparatus according to Embodiment 1 of the present disclose;

FIG. 2 shows a diagram that explains a pixel that the filter processingunit 1 shown in FIG. 1 refers to for filter calculation of a targetpixel;

FIG. 3 shows a diagram that explains a filter coefficient used in thefilter processing unit 1 of the image processing apparatus shown in FIG.1; and

FIG. 4 shows a block diagram that indicates a configuration of an imageprocessing apparatus according to Embodiment 2 of the present disclose.

DETAILED DESCRIPTION

Hereinafter, embodiments according to an aspect of the presentdisclosure will be explained with reference to drawings.

Embodiment 1

FIG. 1 shows a block diagram that indicates a configuration of an imageprocessing apparatus according to Embodiment 1 of the present disclose.The image processing apparatus shown in FIG. 1 generates and outputs abinarized output pixel value corresponding to an input pixel value, andfor example, is included in an image forming apparatus such as aprinter, a copier, or a multi function peripheral, which includes aprinting device that outputs an image having the output pixel value.This image processing apparatus is configured with a computer, ASIC(Application Specific Integrated Circuit), and/or the like. The inputpixel value is a value corresponding to one of multi gradation levelssuch as an 8-bit value, and takes any value in a range from 0 to 1. Theoutput pixel value takes any one of 0 and 1.

The image processing apparatus includes a filter processing unit 1, aquantizer 2, an adder 3, a subtractor 4, and a buffer 5. To this imageprocessing apparatus, the input pixel value of a target pixel isinputted sequentially along a predetermined scanning order.

The filter processing unit 1 performs a bandpass filter process on thebasis of differences (mentioned below) stored by and in the buffer 5.Specifically, the filter processing unit 1 performs an operation for sumof products between the aforementioned differences of adjacent pixels toa target pixel and filter coefficients.

The quantizer 2 quantizes a pixel value after a filter process performedby the filter processing unit 1 and thereby generates the output pixelvalue. Here, a threshold value is set as 0, and in accordance with thethreshold value, if an output of the filter processing unit 1 is equalto or larger than 0, then the output pixel value is set as 1, andotherwise if not, then the output pixel value is set as 0. If the inputpixel value of the target pixel is 0 or 1, then the quantizer 2 sets theoutput pixel value of the target pixel as 0 or 1, that is, such that theoutput pixel value is set to be equal to the input pixel value.

The adder 3 calculates a sum of two pixel values: the input pixel valueof the target pixel and the pixel value processed by the filterprocessing unit 1.

The subtractor 4 calculates for the target pixel a difference between avalue of the calculation result by the adder 3 and the output pixelvalue. This difference is calculated after the binarized output pixelvalue (0 or 1 as a one-bit value) is converted to one of multi-gradationvalues (0 or 1 as a multi-bit value).

The buffer 5 includes a memory area, for example, of a memory such asSRAM (Static Random Access Memory) or DRAM (Dynamic Random AccessMemory), and sequentially stores a value of the calculation result bythe subtractor 4 for a target pixel into the memory area. Thus, for eachpixel, the buffer 5 stores a difference between (a) a sum of the pixelvalue after the filter process by the filter processing unit 1 and theinput pixel value and (b) the output pixel value.

Specifically, the buffer 5 stores the difference corresponding to theoutput pixel value already generated in the aforementioned predeterminedscanning order, and the filter processing unit 1 performs the bandpassfilter process for a target pixel on the basis of the difference alreadystored by the buffer 5 at a time point that the bandpass filter processis performed for this target pixel.

FIG. 2 shows a diagram that explains a pixel that the filter processingunit 1 shown in FIG. 1 refers to for filter calculation of a targetpixel.

Here the target pixel is selected so as to move along the primaryscanning direction and the secondary scanning direction (or meanderingscanning) and therefore the calculation of the output pixel value hasnot been performed for a subsequent pixel after the current targetpixel. Therefore, as shown in FIG. 2, the filter processing unit 1performs the operation for sum of products between the aforementioneddifferences that have been derived and the filter coefficients, withinthe adjacent pixels to the target pixel. In other words, withconsidering the difference on the subsequent pixel as zero, theoperation for sum of products is performed between the aforementioneddifferences and the filter coefficients.

In Embodiment 1, this bandpass filter process has a bandpasscharacteristic obtained by a difference between two Gaussian functions,that is, a DoG (Difference of Gaussian) filter process.

Further, in Embodiment 1, filter coefficients in the bandpass filterprocess are products of (a) the difference between two Gaussianfunctions and (b) correction coefficients, and the correctioncoefficients have different distributions in a primary scanningdirection and in a secondary scanning direction.

In this bandpass filter process, the operation for sum of products isperformed between the aforementioned differences and a filtercoefficient matrix in a filter window having a predetermined size.

FIG. 3 shows a diagram that explains a filter coefficient used in thefilter processing unit 1 of the image processing apparatus shown in FIG.1.

In this embodiment, the filter coefficient “Coefficient (x, y)” of thebandpass filter process is obtained by normalizing a product of (a) thefilter coefficient “DoG (x, y)” of a DoG filter as a difference betweenthe aforementioned two Gaussian functions and (b) the correctioncoefficient “Gain (x, y)”. Here, “x” and “y” indicate distances (thenumber of pixels) from a center of the filter matrix (i.e. a targetpixel) in the primary scanning direction (i.e. x direction) and in thesecondary scanning direction (i.e. y direction), respectively.

The filter coefficient DoG (x, y) of the DoG filter has a valuecalculated by a formula shown in FIG. 3, for example.

Further, the correction coefficient Gain (x, y) has a value calculatedby a formula shown in FIG. 3, for example.

A constant “α” is an adjustment coefficient that takes a value largerthan 0 and less than 1. Vx is a norm of a correction coefficient vectorin the primary scanning direction, Vy is a norm of a correctioncoefficient vector in the secondary scanning direction, and thecorrection coefficient Gain (x, y) is a norm of a vector obtained byvector synthesis of the correction coefficient vectors in the primaryand secondary scanning directions. It should be noted that if y=0 thenVx is set as 0 and if x=0 then Vy is set as 0.

As mentioned, gain distributions of the DoG filter is adjusted such thatthe gain distributions in the primary and secondary scanning directionsget different from each other.

The following part explains a behavior of the image processing apparatusin Embodiment 1.

For a color plane of each component color in one image, this imageprocessing apparatus selects a target pixel in turn along apredetermined scanning pattern, and derives an output pixel value of thetarget pixel in turn.

In this process, for a target pixel, an output pixel value is derivedand outputted by the filter processing unit 1 and the quantizer 2 on thebasis of the aforementioned differences already derived and stored bythe buffer 5, and the aforementioned difference is calculated for thetarget pixel by the adder 3 and the subtractor 4 and stored by and inthe buffer 5.

A value to be stored by and in the buffer 5 is a value obtained byadding the input pixel value to a difference between an input value andan output value of the quantizer (i.e. a quantization error), andtherefore, the filter processing unit 1 substantially performs both thefilter calculation for the input pixel value and the filter calculationfor the quantization error, and thereby a sum of results of the bothfilter calculation is outputted from the filter processing unit 1.

In other words, the filter processing unit 1 as only one filterprocessing unit performs at once both a filter process for input pixelvalues of the adjacent pixels and a filter process for quantizationerrors of the adjacent pixels, and thereby the former realizesclustering output dots and the latter realizes gradation preservationthrough the quantization (corresponding to the error diffusion).

In the aforementioned manner, for all component colors (e.g. CMYK),after obtaining binary images that has the aforementioned output pixelvalue, these binary images are developed with toner of respectivecolors, and subsequently outputted as a color output image.

As mentioned, in Embodiment 1, the quantizer 2 quantizes a pixel valueafter a filter process performed by the filter processing unit 1 andthereby generates the output pixel value. The buffer 5 stores for eachpixel a difference between (a) a sum of the pixel value after the filterprocess by the filter processing unit 1 and the input pixel value and(b) the output pixel value. Further, the filter processing unit 1performs a bandpass filter process on the basis of the aforementioneddifferences stored by and in the buffer 5.

Consequently, clustering output dots is realized with a relatively smallmemory area. Specifically, as mentioned, only the buffer 5 (as only onebuffer) keeps values of the past target pixels, and therefore, an onlyrelatively small memory area is required for the apparatus; and inaddition, performing the filter process one time accomplishes theclustering and the gradation preservation, and therefore, a datacalculation amount and a circuit scale are restrained.

Embodiment 2

FIG. 4 shows a block diagram that indicates a configuration of an imageprocessing apparatus according to Embodiment 2 of the present disclose.As shown in FIG. 4, the image processing apparatus in Embodiment 2includes a noise generating unit 21, an offset control unit 22, and anadder 23, additionally to the image processing apparatus in Embodiment1.

The noise generating unit 21 generates a pseudorandom number, forexample, using a pseudorandom number generator, and generates apredetermined noise on the basis of the pseudorandom number. Forexample, the noise generating unit 21 makes an amplitude of the noisesmaller for a higher density of the target pixel. Consequently, bandingon an intermediate gradation area is restrained in the image.

The offset control unit 22 generates a predetermined offset value inaccordance with an input pixel value of the target pixel such thatadding the offset value to the aforementioned noise restrains unintendedcreation of a dot.

For example, if the input pixel value of the target pixel is in apredetermined range adjacent to the smallest value (i.e. 0 of 256gradation levels), then the offset control unit 22 sets the offset valueas a negative value; and if the input pixel value of the target pixel isin a predetermined range adjacent to the largest value of 256 gradationlevels (255 of 256 gradation levels), then the offset control unit 22sets the offset value as a positive value. For example, the offsetcontrol unit 22 determines the offset value using a lookup table thatoutputs an offset value corresponding to an input pixel value of thetarget pixel.

The adder 23 adds the aforementioned noise and the aforementioned offsetvalue to the output value of the filter processing unit 1.

Other parts of the configuration and behaviors of the image processingapparatus in Embodiment 2 are identical or similar to those inEmbodiment 1, and therefore not explained here.

It should be understood that various changes and modifications to theembodiments described herein will be apparent to those skilled in theart. Such changes and modifications may be made without departing fromthe spirit and scope of the present subject matter and withoutdiminishing its intended advantages. It is therefore intended that suchchanges and modifications be covered by the appended claims.

What is claimed is:
 1. An image processing apparatus that generates andoutputs a binarized output pixel value corresponding to an input pixelvalue, comprising: a filter processing unit; a quantizer configured toquantize a pixel value after a filter process performed by the filterprocessing unit and thereby generate the output pixel value; and abuffer configured to store for each pixel a difference between (a) a sumof the pixel value after the filter process and the input pixel valueand (b) the output pixel value; wherein the filter processing unitperforms a bandpass filter process on the basis of the difference storedby the buffer.
 2. The image processing apparatus according to claim 1,wherein the input pixel value is sequentially inputted along apredetermined scanning order; the buffer stores the differencecorresponding to the output pixel value already generated in thepredetermined scanning order; and the filter processing unit performsthe bandpass filter process for a target pixel on the basis of thedifference already stored by the buffer at a time point that thebandpass filter process is performed for this target pixel.
 3. The imageprocessing apparatus according to claim 1, wherein the bandpass filterprocess has a bandpass characteristic obtained by a difference betweentwo Gaussian functions.
 4. The image processing apparatus according toclaim 3, wherein filter coefficients in the bandpass filter process areproducts of (a) the difference between two Gaussian functions and (b)correction coefficients; and the correction coefficients have differentdistributions in a primary scanning direction and in a secondaryscanning direction.
 5. An image forming apparatus, comprising: an imageprocessing apparatus configured to generate and output a binarizedoutput pixel value corresponding to an input pixel value; wherein theimage processing apparatus comprises: a filter processing unit; aquantizer configured to quantize a pixel value after a filter processperformed by the filter processing unit and thereby generate the outputpixel value; and a buffer configured to store for each pixel adifference between (a) a sum of the pixel value after the filter processand the input pixel value and (b) the output pixel value; wherein thefilter processing unit performs a bandpass filter process on the basisof the difference stored by the buffer.